The physiological role of TRP channels in sleep and circadian rhythm

Abstract TRP channels, are non‐specific cationic channels that are involved in multiple physiological processes that include salivation, cellular secretions, memory extinction and consolidation, temperature, pain, store‐operated calcium entry, thermosensation and functionality of the nervous system. Here we choose to look at the evidence that decisively shows how TRP channels modulate human neuron plasticity as it relates to the molecular neurobiology of sleep/circadian rhythm. There are numerous model organisms of sleep and circadian rhythm that are the results of the absence or genetic manipulation of the non‐specific cationic TRP channels. Drosophila and mice that have had their TRP channels genetically ablated or manipulated show strong evidence of changes in sleep duration, sleep activity, circadian rhythm and response to temperature, noxious odours and pattern of activity during both sleep and wakefulness along with cardiovascular and respiratory function during sleep. Indeed the role of TRP channels in regulating sleep and circadian rhythm is very interesting considering the parallel roles of TRP channels in thermoregulation and thermal response with concomitant responses in growth and degradation of neurites, peripheral nerves and neuronal brain networks. TRP channels provide evidence of an ability to create, regulate and modify our sleep and circadian rhythm in a wide array of physiological and pathophysiological conditions. In the current review, we summarize previous results and novel recent advances in the understanding of calcium ion entry via TRP channels in different sleep and circadian rhythm conditions. We discuss the role of TRP channels in sleep and circadian disorders.

Combined, TRP channels play a regulatory role that is controlled by the environmental factors that effect TRP channels.Regulatory control of sleep and circadian rhythm through TRP channels shows how the brain is dynamically modified on a cellular level based on environmental experiences that can then again be reshaped for better or for worse.Here we look at various sleep and circadian disorders and how they are regulated on a cellular and molecular level by TRP channels.These findings, in turn, reinforce neuroscientists to perceive efficacious therapeutics that support these biological explanations of how sleep and circadian rhythm is regulated.

| OVERVIE W OF THE MAMMALIAN TRP CHANNEL FAMILY
TRP proteins are six transmembrane domain-containing subunits that form homo-or heterotetrameric non-selective cation channels.
The TRP superfamily includes at least 28 related channels that play an important role in several cellular functions ranging from sensory transduction (including invertebrate vision, temperature, pain and gustatory and osmolarity detection) to development.The first member of the TRP superfamily was identified as a protein involved in phototransduction in Drosophila. 1 The trp gene was named on the basis of the transient, rather than sustained, receptor potential observed in response to light in mutant flies.From the beginning a relationship between TRP proteins and ionic currents across the membrane was suggested since TRP mutants displayed a defect in light-induced Ca 2+ influx, which together with the predicted structure of TRP and the related protein, TRPL, raised the possibility that these proteins were Ca 2+ influx channels. 2,3P proteins are present in yeast, Drosophila, Caenorhabditis elegans, fish and mammals.TRP channels are widely expressed in both excitable and non-excitable cells, where they have been reported to mediate Ca 2+ entry.Although all TRP proteins form cation channels, they differ significantly in their cation permeability and activation mechanisms, although most members of the TRP superfamily share significant sequence homology.TRP channels can be grouped into six subfamilies: those most closely related to Drosophila TRP (TRPC, TRPV and TRPM), two subfamilies that are more distantly related to Drosophila TRP (TRPP and TRPML), and a less related TRPN group that is absent in mammals but expressed in flies and worms and includes the mechanosensory channel NOMPC. 2,3The TRPC subfamily groups the mammalian proteins that display the greatest similarity to Drosophila TRP, sharing between 32% and 47% amino acid homology over the N-terminal region.TRPC proteins show the prototypical structure, including three or four ankyrin repeats, the six transmembrane domains, and a highly conserved 25 amino acid sequence known as the TRP box, a hydrophobic region located just C-terminal to the sixth transmembrane domain.TRPV proteins also include three or four ankyrin domains as well as the TRP box, and TRPM proteins contain a TRP box, but no ankyrin repeats, and some members, such as TRPM6 and TRPM7 exhibit a C-terminal kinase domain. 3st TRP channels are non-selective for monovalent and divalent cations with wide range of Ca 2+ to Na + permeability ratios.
Especially relevant are TRPM4 and TRPM5, which are selective for monovalent cations, as well as the Ca 2+ -selective members TRPV5 and TRPV6 that exhibit a Ca 2+ to Na + permeability ratio over 100.Ca 2+ and Na + influx through TRP channels leads to membrane depolarization while increasing cytosolic Ca 2+ and/or Na + concentrations, 4 thus reducing the driving force to Ca 2+ influx to other Ca 2+ channels.This article presents an overview of what is currently know of the molecular relationship between TRP channels and their role in physiological cell processes that regulate sleep and circadian rhythm.

| TRP CHANNEL S REG UL ATE S LEEP AND CIRC AD IAN RHY THM BY S EN S ING LI G HT AND HE AT
Temperature and light modulate our circadian rhythm by a mechanism involving TRP channels and clock genes. 5,6The light information recorded through the retina is translated into the physiological circadian response, [7][8][9][10] thus, the organism is able to respond to differences in environment light and temperature to maximize cellular energy and metabolic resources efficiently throughout the day. 11,12It was shown that temperature plays a highly significant role in regulating our sleep and circadian rhythm through multiple clock genes and TRP channels. 13The modulation of the circadian rhythm by temperature is mediated by a mechanism involving the activation of rhodopsin and TRP channels. 14is is a molecular mechanism whereby external light and temperature are able to communicate sleep and wakefulness through multiple clock genes 15 and TRP channels located throughout the body including muscle cells, neuronal cells and peripheral nerves sending cues as to time of day. 16,177][28][29][30][31] Indeed, light sensing retina expresses TRPC6 10,32-34 and TRPC7, 33,34 whereas, changes in heat are sensed by TRPV1 [35][36][37][38][39][40][41][42] and changes in cold are sensed by TRPM8. 12,36,434][55][56] The suprachiasmatic nucleus, located in the hypothalamus, is responsible for regulating the effects of temperature change upon sleep and the circadian rhythm. 57Despite suprachiasmatic nucleus-projecting retinal ganglion cells act independently and separately from photoreceptors, the retinal ganglion cells express TRP channels which behave similarly as in photoreceptors upon light stimulation. 58Indeed, it has been hypothesized that the modulation of the circadian rhythm is not solely delegated to photoreceptors.The period (Per) and timeless (tim) genes have been shown to regulate circadian rhythm and sleep in Drosophila. 59NinaE fruit flies that genetically lack the gene and protein (rh1) for the rhodopsin receptors was able to show sleep, wakefulness and circadian rhythm behaviour in the absence of photoreceptors. 59In addition, when Drosophila flies were mutated for trp and trpl genes their visual transduction cascade was impaired, which attenuated the tim gene response to light, but circadian rhythm behaviour was only partially disturbed, thus suggesting that the circadian rhythm does not rely on the visual system but has its own independently dedicated system for photoreception. 59

| TRP CHANNEL S REG UL ATING ME TABOLIS M PL AY A ROLE IN S LEEP AND CIRC ADIAN RHY THM
Orexins, particularly Orexin A and Orexin B, have been shown to be involved not only in insulin secretion and energy metabolism, but also in the sleep-wake circadian rhythm cycle.It has been reported that blockade of TRP channel-mediated calcium release by lanthanum abrogates orexin activity, thus suggesting that TRP channels are involved in the mediation of orexin functions. 60Indeed, fat metabolism has been shown to play a role in regulating the circadian rhythm.TRP channels have been shown to regulate the circadian rhythm, sleep wake cycle, along with fat and energy metabolism in brown adipose tissue. 12TRPM8, known as a cold temperature sensing receptor, was found to regulate the clock circadian rhythm genes in brown adipose tissue along with the circadian rhythm gene Per1.Clock and Per1 genes amplitude and oscillation were found to be reduced both in the eyes and brown adipose tissue of TRPM8 knockout mice.Similarly, UCP1, a mitochondrial membrane protein essential in brown fat metabolism was greatly reduced. 12Heat sensitive TRPV1 receptors were shown to alter matrix metalloprotein expression independent of clock genes in the eye. 36TRPA1 was found to regulate sleep and circadian rhythm in response to environmental temperature. 61TRPA1 was the first thermosensing TRP channel to be described in invertebrates associated to the regulation of the rhythmic sleep-wake changes in body temperature: cooler when sleeping and warmer when awake. 61Melanopsin receptors, that regulate the duration of the sleep-wake cycle in response to light in certain organisms, have been shown to mediate their effects on the sleep-wake cycle through TRP channels activation, particularly in amphioxus. 62Temperature sensing TRP channels have been shown to environmentally synchronize the cold-warm body temperature changes that correspond with the sleep-wake circadian rhythm cycle.Drosophila flies lacking the Pyrexia gene, a TRP channel found in the fruit fly, have been found to be unable to synchronize their behaviour to temperature cycles in the lower range (between 16 and 20°C), which provide further evidence for the involvement of TRP channels in the synchronization of the circadian rhythm by temperature. 63

| TRP CHANNEL S A SSO CIATI ON WITH S LEEP AND CIRC AD IAN RHY THM G ENE S
The TRP channel, TRPA1, was shown to control arousal from sleep as TRPA1 knockout mice lack of arousal from sleep when exposed to noxious formalin odours. 64The circadian rhythm in Drosophila regulates colour discrimination and preference with TRPA1 controlling the preference of dim light over the colour green during the midday and avoidance of blue light during the day, which is controlled through rhodopsin 7 and the Drosophila TRP channel Painless found in multi-dendritic neurons. 65role for TRP channels in bladder function has also been described to be regulated by clock genes.An increase in gene expression for TRPV1, TRPV4, Piezo1, and VNUT in the spontaneously hypertensive rat (SHR) was attributed to having a regulatory role over circadian genes Cry2 and Clock in the SHR bladder resulting in greater number of urination times during day and night cycles but a lower urination volume. 66Bladder function, including frequency of urination, day or night occurrence of urination and volume, are all regulated by circadian clock genes such as Per2.One interesting study looked at the circadian gene regulation of bladder function under conditions of stress.A drug that inhibits Per2 phosphorylation, PF670462 (PF), was able to correct irregular stress-induced clock gene expression along with sensory bladder fullness genes, such as TRPV4, Piezo1 and Connexin26 in restoring normal circadian gene control of bladder function. 67Another study has reported that the expression of mechanosensory, such as Piezo1 and TRPV4, and main ATP release pathways, including Connexin26 and vesicular nucleotide transporter (VNUT), are regulated by clock genes in the bladder mucosa, thus, the expression of these genes is low in the sleep phase and modulating the frequency of urination during sleep. 68 has been reported that surgical removal of the mutant temperature sensing TRPA1 gene called Pyrexia or Pyx, from the antennae of Drosophila restores normal circadian rhythm of mechanosensory neurological function of body positioning (proprioception) and hearing through the circadian protein called PERIOD. 69osophila fruit fly has 13 different TRP channels, 9 that are directly involved in regulating the circadian rhythm cycle.Among them, TRP, TRPL, Inactive, Brivido-1, Brivido-2, Brivido-3 are all Drosophila TRP channels that play a circadian rhythm role in thermotaxis and locomotion in direct reaction to cool temperatures, whereas, the dTRPA1 provides circadian rhythm locomotion response to warmer temperatures, fluctuation in temperature and avoiding toxic or noxious heat.Finally, Painless and Pyrexia TRP channels are involved in noxious heat avoidance and regulating the circadian rhythm cycle in response to fluctuations in temperature. 70Drosophila neurons that express the heat sensing TRP channel, dTRPA1 have been shown to regulate motor activity of the sleep/wake cycle corresponding to light, day/night cycle, in fruit flies. 71The diurnal/daily intestinal motor activity found in gastrointestinal reflux disease was shown to be regulated both by TRPV1 channels along with the circadian rhythm genes Per1, Per2, BMAL1 and CRY2, TRPV1, along with NGF. 72cently, it was discovered that during the afternoon, not just morning or evening (siesta behaviour), locomotive behaviour such as seeking a shaded location from the warm sun, is also regulated by the Drosophila heat thermosensing dTRPA1 channel. 73Neonatal rats that were treated with capsaicin had TRPV1 receptors desensitized to heat but had an inverse to normal circadian rhythm body temperature cycle and circadian, Hsf1 and Per2, gene expression. 37Mutant TRPA1 Drosophila were shown to have shorter morning activity with evening activity occurring later than normal under circadian cycle light conditions and at 18 degrees centigrade. 74The thermosensing TRP receptor Pyrexia was found in peripheral sensing chordotonal organs of Drosophila where they synchronize temperature with the circadian rhythm clock genes period (Per) and timeless (tim) forming a negatively regulated feedback loop with circadian transcription factors clock (clk) and cycle (cyc). 63Drosophila TRP channel, TRPA1 is found in the pacemaker neurons of the Drosophila brain where it is found to regulate the 2-3°C decline in temperature during the circadian sleep/wake cycle of the fruit fly. 75Similar to TRP channels, another calcium permeable ion channel located in the suprachiasmatic nucleus of the brain are the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels that have also been shown to regulate the circadian rhythm gene Per2. 76The cytokine IL-15 in the hypothalamus has been reported to regulate both metabolism and temperature in a circadian rhythm fashion in participation with TRPV4. 38Likewise, somatostatin has been shown to alter the function of nucleotide gated ion channels in their response to circadian rhythm genes and light. 77

| THE ROLE OF TRP CHANNEL S IN S LEEP APNE A
TRP channels have also been found to be involved in sleep apnea.
TRPC5 has been found to be highly elevated in obstructive sleep apnea suggesting that calcium entry through this channel might play a role in the myocardial damage that occurs in obstructive sleep apnea. 78Inflammatory mediator regulation of TRP channels by circulating exosomal miRNAs has been shown to play an important role in abnormal circadian regulation of blood pressure and could also serve as an indicator of the risk of cardiovascular disease associated with obstructive sleep apnea. 79Sleep deprivation has been shown to cause dry eye by producing unusual microvilli formation in superficial corneal epithelial cells that has been linked to low levels of TRPV6 expression. 80External environmental temperature has been shown to play a large role in regulating sleep duration and sleep circadian rhythm behaviour through the Drosophila TRPA1 channels located in the neuronal circuits. 61N-acyltaurine is a fatty acid amide that activates and acts as an agonist for TRP channels.Fatty acid amide hydrolase (FAHH) is an enzyme that hydrolyses fatty acid amides.N-arachidonoyl-serotonin (AA-5-HT) is a TRPV1 channel blocker that is shown to alter the sleep and circadian rhythm cycle when administered at the start of the dark period resulting in a lack of wakefulness and heightened slow wave sleep along with an increase in the rapid eye movement sleep phase. 81TRP channels have been shown to be upregulated in the tobacco hornworm, Manduca sexta during the quiescent state characteristic of the moult. 82Obstructive sleep apnea was shown to increase the sensitivity of posterior cerebral arteries to the vasoconstrictor endothelin-1 through elevated endothelin-B receptor activity, and increased activation of TRPC receptors and Rho kinase.Excessive vasoconstriction of posterior cerebral arteries associated to obstructive sleep apnea was alleviated through use of the TRPC receptor antagonist SKF96365. 83Likewise, elevated expression of ion channel proteins was found in chronic obstructive sleep apnea with remodelling of the cardiac atrium. 84

| ROLE OF TRP CHANNEL S IN CONTROLLING ALERTNE SS AND PRE VENTING S LEEP
TRPA1 knockout mice were found to be unresponsive in the fight or flight response noxious formalin odours.In fact, these mice were shown to sleep completely through exposure to toxin odours that were found to have caused massive effect on the brain as measured by c-fos expression 64 Spinal D-amino acid oxidase was shown to induce sleep derived mechanical pain sensitivity through production of hydrogen peroxide, a direct pain inducing agonist of the TRPA1 nociceptive receptor. 85Studies expressing the temperature-gated TRPA1 in Drosophila neurons to induce sleep on demand have reported that sleep facilitate consolidate memory in Drosophila. 86cently it was shown that peripheral sensory organs contribute to temperature synchronization of the circadian clock in a cell autonomous mechanism that involves TRP channels 87 TRPM4 channels were found to be expressed in circadian associated pacemaker LC and SCN neurons, and that TRPM4 contributes to subthreshold oscillations observed in those cells in neonatal mouse brainstem slices. 88P channels, as described in this review, play a significant role in regulating activity revolved around the quality and duration of sleep corresponding to the circadian rhythm cycle.TRP channels likewise form a systemic neurosensory chain orchestrating the relationship between sleep and circadian rhythm.TRP channels clearly show that sleep is important for long-term memory and that even memory and control of temperature regulation during sleep along with memory of pain that effects sleep and wakefulness all intricately depend on coordination with TRP channels.

AUTH O R CO NTR I B UTI O N S
Geoffrey Woodard: Writing -original draft (lead); writing -review and editing (lead).He Li: Conceptualization (lead); supervision